The present invention concerns the field of land-vehicle suspensions, and more precisely means used to resist body roll or body pitch. In a particular application, it relates to the suspension of passenger cars.
It is known that the suspension of a vehicle embodies constructional arrangements by means of which the displacement of a wheel or of wheels of an axle with respect to the body (or with respect to the chassis if the vehicle is provided with a chassis, or between body and chassis if appropriate) is guided, and it includes means controlling the deflection of each wheel (a wheel is a member belonging to non-suspended members) with respect to the body (any point situated on the body is referred to as a xe2x80x9csuspendedxe2x80x9d point), in order to bear the load while maintaining a certain ground clearance of the vehicle (this is the function of a spring) and in order to prevent the maintenance of the oscillations arising between suspended and non-suspended points following a disturbance (this is the function of a damper).
It is known that the design of a suspension is characterised by the search for a compromise between objectives which have a reputation for being, at least to a large extent, contradictory: comfort and handling. Numerous efforts have been made to find increasingly favorable compromises. In the state of the art, suspension systems are known which are referred to as active or semi-active suspensions and whose control has now been made possible by progress in electronics. In such suspensions, commands are transmitted by electric or electrohydraulic means in order to modify the flexibility and/or damping characteristics of the suspension in real time. This leads to the addition of an electronic regulation stage, with its sensors and means for calculation, to conventional mechanical, pneumatic or hydraulic devices, as well as solenoid valves or servovalves, in order to transmit the commands for changing the characteristics to the mechanical members responsible for the functioning of the suspension as such. For example, a valve in a damper has to be opened or closed in order to modify the dissipation of energy which it brings about.
In the majority of suspension systems for passenger cars, trucks and passenger-transporting coaches, there also exist specific means for opposing the roll of the body (or chassis) under the effect of centrifugal forces. The tilting of the body about a horizontal axis contained in a longitudinal and vertical plane of symmetry of the vehicle is called xe2x80x9crollxe2x80x9d. Excessive roll is detrimental to the comfort of the passengers and, depending on the wheel-guiding means used, may even be detrimental to the working of the tires when the camber of the tire with respect to the ground is influenced by the roll of the body. This is typically the case for a double-wishbone suspension, which is widely used in passenger cars. The roll then influences the stresses to which the said tire is subjected and the more or less favorable position in which it is situated for transmitting forces, in particular forces in the transverse direction. It is known that these forces are of prime importance for the guidance of the vehicle, and thus for safety.
The most commonly employed means for opposing this roll consists in using an anti-roll bar, in general one anti-roll bar per axle. Unfortunately, an anti-roll bar can only limit the roll by opposing the latter with a reaction torque. On principle, it cannot prevent the roll, and thus it cannot prevent the tilting of the wheels in a direction unfavorable to the correct functioning of the tires. In addition, it is known that an anti-roll bar needlessly stiffens the suspension in a straight line, since it opposes the non-identical suspension deflections of the wheels of an axle connected by such a bar. It is only in a pure vertical deflection that the anti-roll bar does not intervene.
From the patent application WO 99/67100, a proposal is already known which describes an active roll control, produced by means of an anti-roll bar comprising an electric motor intended to impose thereon a pre-tension in the desired direction. However, this solution is limited to the control of the roll.
The pitch of a vehicle (lift at the front on acceleration, diving on braking) poses problems comparable to those posed by roll. The pitch is generally opposed by suspension configurations adapted so that the longitudinal transfers of loads only partially stress the springs, but this is a palliative not without detrimental effects. The roll and pitch are both referred to hereinbelow as xe2x80x9ctrim variation xe2x80x9d. A trim variation embraces, in the present specification, a variation of the ground clearance and also any tilting with respect to the a reference plane linked to the non-suspended mass of the vehicle.
Also known are numerous proposals for active suspensions, such as those described in the patents or patent applications U.S. Pat. Nos. 5,060,959, 5,028,073, 5,066,041, 5,740,039 or EP 0 878 333, to mention only some of them. The diversity of the constructional arrangements and of the control laws proposed compared with the very small number of active suspensions on the market gives an idea of the difficulty of controlling the suspensions effectively. The state of the art comes up against the choice of control parameters, and the correct utilization of a multitude of signals which are thought to be needed for correctly capturing the attitude of a vehicle.
The aim of the invention is to obtain a high level of operational safety of the vehicles and great comfort.
The invention proposes a vehicle suspension device comprising at least two suspension systems each connecting a non-suspended mass comprising a wheel to a suspended mass with possible vertical deflection, each suspension system comprising a vertical suspension reacting to the variations of distance between the suspended mass and the non-suspended mass, the said vertical suspension comprising a spring and a reversible electric actuator acting in parallel with the spring, the device comprising means for evaluating a non-vertical stress applied to the suspended mass and likely to bring about a trim variation of the suspended mass, the said device comprising means for controlling the electric actuators of each of the said suspension systems so that they each develop a force consisting of the addition of two components: a damping component opposing the deflection speed and a trim correction component, the said trim correction components applying a trim correction to the suspended mass.
Thus, through the simple programming of the mere two components indicated above, one of which is a dissipative damping component, it is possible to control the force in the electric actuator so as to obtain a safe and relatively comfortable suspension. The present proposal does not have recourse to more complex means as proposed in the patent application EP 0 878 333, in which a road-contour-following control is proposed, accompanied in one embodiment by a detection of the occurrence of a resonance of the tire in order to introduce a damping.
An advantageous example of the use of the invention concerns a trim correction controlled by means for evaluating a non-vertical stress applied to the suspended mass and likely to bring about a trim variation of the suspended mass. The rest of the description concerns essentially roll, since this is sufficient to make the invention clear. The said at least two suspension systems form an axle and are mounted on either side of the median plane of symmetry of the vehicle, so that the said trim correction torque opposes the roll. However, this is not limiting. All that has been said in connection with roll may be developed in connection with pitch. The invention can be applied to a bicycle such as a motorcycle. The proposed means can be used to oppose either or both of these phenomena. The invention thus extends to the case in which the said at least two suspension systems comprise a front suspension system and a rear suspension system, so that the said trim correction torque opposes the pitch, and in particular to the case in which the vehicle has at least four suspension systems forming a front axle and a rear axle, in which the said trim correction torque opposes both the roll and the pitch.
Of course, if both the roll and the pitch are compensated, each of the said suspension systems is capable of receiving from the means for controlling the electric actuators a force value which is the resultant of a contribution for the correction of the roll and of a contribution for the correction of the pitch. For each electric actuator, the overall trim correction component is the algebraic sum of a contribution relating to the roll and a contribution relating to the pitch. In the case of pure roll (no pitch) or pure pitch (no roll), the correction components are equal absolute values and are oriented in opposite directions to one another on either side of the vehicle, that is to say on the left-hand side and on the right-hand side as far as the roll is concerned, and at the front and rear as far as the pitch is concerned.
The invention proposes an approach which, having observed that the control of the roll is the determining factor both for the feeling of comfort and safety experienced by the passengers and for the correct functioning of the tires, uses a spring (mechanical or pneumatic) and a reversible electric actuator acting in parallel with the spring as elements for controlling the deflection of a suspension system. The electric actuators of certain suspension systems, and preferably of all the suspension systems of the vehicle, are controlled so as to compensate at least partially the roll or the pitch, while providing the possibility of introducing a variable damping. In addition, it should be pointed out that it may be advantageous to overcompensate either of the trim variation phenomena. It is possible, for example, to overcompensate roll on a bend, that is to say bring about a tilting of the body towards the inside of the bend, if only by a fairly limited angle.
According to a preferred embodiment, the invention uses a conventional spring, for example a helical spring, which is dimensioned and arranged to bear the static load of a vehicle and provide a predetermined ground clearance. The value of its flexibility according to the deflection is constant or variable.
It is known that, in the case of very flexible suspensions designed to promote comfort, generally considerable roll and pitch are experienced which very often bring the suspensions into an operating area close to a deflection stop. In the prior art, these undesirable phenomena were opposed by acting on the suspension geometry to oppose the pitch and by using anti-roll bars. Here, in contrast, these phenomena are opposed by assisting the spring of each suspension system in an appropriate manner. This makes it possible to use springs of much greater flexibility without encountering disadvantages inherent in suspensions which are too flexible.
It has been indicated above that the control of the trim can also include the control of the ground clearance, for example in order to obtain a constant ground clearance irrespective of the static load of the vehicle. As already indicated in the patent application EP 0 878 333, to make a height correction it must be accepted that energy is continuously consumed in order that the fraction of the load exceeding the dead weight of the vehicle is borne by the electric actuator. The reason is that this assumes that a fraction of the trim correction component continuously opposes the said load exceeding the dead weight, which is not incompatible with the principle according to the invention, but requires the establishment of a constant current component in the electric actuators. However, it seems preferable to do without height correction, or to reserve the height correction function for a device other than the electric actuator, this other device acting with a great time constant and consuming energy only during the correction phase.
The invention will be better understood from the following description of an example given on a non-limiting basis, with reference to the appended drawings.